Add roster members

Add customers to a Discovery campaign's roster. They join this month's calendar, so its next run interviews them, and their conversations accrue against the organization's plan. Refused on a campaign that is not active or paused or is a one-time campaign whose run has finished, while its first run is still choosing its customers, when any customer cannot join (each named with its reason), or when Billing would not admit the work; read discovery_roster_quotes_get first. Returns how many joined and when they start.